Eggs of different sizes/shades...can they be from the same hen?

We have a flock of 18 birds, with 15 hens. The oldest is 6 months old (breed unknown) , the next 8 are 5 1/2 months old (EEs), and then we've got 5 aracaunas that are almost 4 months old.
We have been finding eggs for about 1 month now, about 2 every three days, in the same place. They have all been brown, but different shades of it, and sometimes with speckles, others plain. Also, the size is very variable...we found one the other day at least 3 times as large as the others...very pointy, as well.
I would think that since the EEs are all the same age, that if they were laying, that we would see a lot more eggs. But, if it is the one hen older than they are, can she lay such variable eggs?
Thank you for your anwers...my children are at the point that they want to install a video camera in the chicken house to find out who is laying these! My 7 year old took an egg around to all of the hens to show them, asking them "is this yours?" I wish i'd had a video camera!
Sincerely,
Jennifer

I have 3 EE hens and the two lay green eggs, but sometimes they are both a soft green color, and sometimes one or both are more khaki green. The blue egg layer sometimes lays them so light they are nearly white, and other times they are nearly robin's egg blue, so I'd have to say there can be some variation. On the other hand, my brown egg layers all lay their eggs exactly the same colors.

Sounds like you have a few hens laying on different days. I've always been able to tell who is laying based on the eggs. The variations are subtle and only change slightly over time (usually just size) as the hens mature.